The present invention generally relates to a communication subsystem and, in particular, relates to such a subsystem having a distributed masterless multi-microcomputer architecture.
The development of communication subsystems evolved as an alternative to central telephone exchanges to avoid the problems of bottlenecking and catastrophic failure of the entire system due to the failure of an element at the central exchange. These problems were exacerbated as non-voice or data systems became more prevalent since such data systems were also centrally based, i.e. by what is commonly known as a central processing unit (CPU). In fact, these problems became compounded due to the inherent characteristics of data communication. For example communication protocol coversions were required, more rapid path set-up and tear-down was required and adaptation for newer and/or different peripherals and/or services became a constant demand. Further, as a result of centralization, the growth of networks in response to an increase in subscribers and/or services became difficult, often requiring an expensive redesign of a substantial portion, if not all, of the system.
For the most part, the above problems were dissipated by the fully distributed ITT SYSTEM 12 digital exchange. In that design the path set up and tear down command and control functions were decentralized. The architecture essentially includes a digital switching network (DSN) having a plurality of terminal control elements (TCE) interconnected thereto and thereby. Each TCE includes the necessary means for path control between subscribers to different TCE's. Consequently, in the event of a failure in the DSN each TCE nevertheless provides full services to the subscribers thereto, and in the event of a failure in any TCE, the remaining TCEs and DSN provide full service to the subscriber thereto. In addition, the provision of alternate, or redundant paths and services can readily be provided and this further reduces any impact of any failure on the subscribers to a particular TCE.
Another significant advantage derived from a fully distributed network, such as the ITT SYSTEM 12 digital exchange, is that, by use of printed circuit board assemblies each TCE can be expanded in a modular fashion. In addition, the number of TCE's interconnected to a DSN can be modularly increased.
Nevertheless, many centralized circuit switched exchanges, large and small, remain in service and are quite unadaptable to provide the present and future data communication needs. As a result, the most common architecture for the provision of data communication services is the local area network (LAN). The LAN is, most frequently, a data network of the private premises network that is installed in addition to presently existing circuit switched exchanges (PABX's).
The problems of centralization are further compounded due to the plethora of different data peripherals presently available as well as those anticipated. Among the major problems are that present and future data peripherals have a variety of communication protocols and data transfer bit rates. In addition, system growth, whether due to increased use by existing subscribers or by an increase in subscribers or an increase in the services to be provided usually entails either a substantial reconfiguration of both the hardware and software or the provision of a new system.